Protective apparel includes coveralls, gowns, smocks and other garments whose purpose is either to protect a wearer against exposure to something in the wearer's surroundings, or to protect the wearer's surroundings against being contaminated by the wearer. Examples of protective apparel include suits worn in microelectronics manufacturing cleanrooms, medical suits and gowns, dirty job coveralls, and suits worn for protection against liquids or particulates. The particular applications for which a protective garment is suitable depends upon the composition of the fabric or sheet material used to make the garment and the way that the pieces of fabric or sheet material are held together in the garment. For example, one type of fabric or sheet material may be excellent for use in hazardous chemical protection garments, while being too expensive or uncomfortable for use in medical garments. Another material may be lightweight and breathable enough for use in clean room suits, but not be durable enough for dirty job applications.
The physical properties of a fabric or sheet material determine the protective apparel applications for which the material is suited. It has been found desirable for a wide variety of protective garment applications that the material used in making the protective garment provide good barrier protection against liquids such as body fluids, paints or sprays. It is also desirable that the material used in making protective apparel block the passage of fine dirt, dust and fiber particles. Another group of desirable properties for fabrics or sheet materials used in protective apparel is that the material have enough strength and tear resistance that apparel made using the sheet material not lose its integrity under anticipated working conditions. It is also important that fabrics and sheet materials used in protective garments transmit and dissipate both moisture and heat so as to permit a wearer to perform physical work while dressed in the garment without becoming excessively hot and sweaty. Finally, most protective garment materials must have a manufacturing cost that is low enough to make the use of the material practical in low cost protective garments.
A number of standardized tests have been devised to characterize materials used in protective garments so as to allow others to compare properties and make decisions as to which materials are best suited to meet the various anticipated conditions or circumstances under which a garment will be required to serve. The strength and durability of sheet materials for apparel have been quantified in terms of tensile strength, tear strength and elongation. The primary test used for characterizing liquid barrier properties is a test of resistance to passage of water at various pressures known as the hydrostatic head resistance test. Particulate barrier properties are measured by bacterial barrier tests and particle penetration tests.
Thermal comfort of fabrics and sheet materials has traditionally been presumed to correspond to the tested moisture vapor transmission rate (MVTR) of the material. However, MVTR is determined under static laboratory conditions, which measure vapor transported by molecular diffusion only. MVTR test results have not proved to be an entirely reliable means of predicting an apparel sheet material's comfort under actual dynamic workplace conditions. In a study of various apparel sheet materials conducted for DuPont by an independent testing laboratory, it was learned that a material's air permeability was the most reliable predictor of the relative comfort afforded by various fabric and sheet materials worn in protective garments. The significant contribution that air permeability makes to the thermal comfort of a garment appears to be due to motion induced pumping of air and moisture through the fabric or material. Because molecular diffusion of water vapor (measured by MVTR) is a relatively slow process, it appears that even small flows of moisture-laden air through a fabric or sheet material can have significantly more impact on moisture vapor transport through a material. Accordingly, it is important that sheet materials used in protective apparel have a high degree of air permeability without unduly sacrificing other important properties such as strength or barrier.
Porous sheet materials are also used in the filtration of gases where the filtration materials are used to remove dirt, dust and particulates from a gas stream. For example, air filters and vacuum cleaner bags are designed to capture dirt, dust and fine particulates while at the same time allowing air to pass through the filter. Porous sheet materials are also used in applications where it is necessary to filter out microbes such as spores and bacteria. For example, porous sheet materials are used in the packaging of sterile medical items, such as surgical instruments. In sterile packaging, the porous packaging material must be porous to gases such as ethylene oxide that are used to kill bacteria on items being sterilized, but the packaging materials must be impervious to bacteria that might contaminate sterilized items. Another application for porous sheet materials with good barrier properties is for making pouches that hold moisture absorbing desiccant substances. Such desiccant pouches are frequently used in packaged materials to absorb unwanted moisture.
The physical properties of a fabric or sheet material determine the filtration applications for which the material is suited. It has been found desirable for sheet materials used in a variety of filtration applications to provide good barrier to the passage of fine particles but also have good permeability to gases. Another set of desirable properties for fabrics or sheet materials used in certain filtration applications is that the material have enough strength and tear resistance that filters made using the sheet material will not lose their integrity under anticipated working conditions. Finally, most filter materials must have a manufacturing cost that is low enough to make the use of the material practical in low cost filters.
A number of standardized tests have been devised to characterize materials used in filtration and in sterile packaging so as to allow others to compare properties and make decisions as to which materials are best suited to meet the various anticipated conditions or circumstances under which a material will be required to serve. The strength and durability of sheet materials has been quantified in terms of tensile strength, tear strength and elongation. The primary tests used for characterizing filtration efficacy are tests that measure filter efficiency (% of particulates retained by a filter); the air permeability for air filters; the resistance to water flow through a filter at a given flow rate for liquid filters (also known as clean permeability); and life of a filter material under a given loading and operation condition (also known as capacity). Barrier properties can be measured by both bacterial or particulate barrier tests.
Tyvek® spunbonded olefin is a flash-spun plexifilamentary sheet material that has been in use for a number of years as a material for protective apparel. E. I. du Pont de Nemours and Company (DuPont) makes and sells Tyvek® spunbonded olefin nonwoven fabric. Tyvek® is a trademark owned by DuPont. Tyvek® nonwoven fabric has been a good choice for protective apparel because of its excellent strength properties, its good barrier properties, its light weight, its reasonable level of thermal comfort, and its single layer structure that gives rise to a low manufacturing cost relative to most competitive materials. DuPont has worked to further improve the comfort of Tyvek® fabrics for garments. For example, DuPont markets a Tyvek® Type 16 fabric style that includes apertures to improve breathability. DuPont has also produced water jet softened Tyvek® fabric (e.g., U.S. Pat. No. 5,023,130 to Simpson) that is softer and more opened up to enhance comfort and breathability. While both of these materials are indeed more comfortable, the barrier properties of these materials are significantly reduced as a consequence of their increased breathability.
In the early 1990's DuPont made a sheet from polyethylene fiber pulp which sheet was designed for use as a filtration media . This sheet was sold under the Hysurf™ mark and was made by a multiple step process disclosed in U.S. Pat. Nos. 5,047,121 and 5,242,546. According to the process, flash-spun polyethylene scrap material was first chopped up and refined to form a pulp. The pulp was mixed with water and surfactants to form a slurry which was then made into a sheet by a wet lay papermaking process. This sheet material was used in vacuum cleaner bags.
International Patent Publication Nos. WO 98/07905 and WO 98/07908 (both assigned to DuPont) disclose flash-spun plexifilamentary sheet material that demonstrates good barrier properties and improved breathability. A number of point bonded and softened plexifilamentary sheet materials disclosed in International Patent Publication No. WO 98/07908 exhibited a Gurley Hill Porosity of about 9 seconds in a sheet that also demonstrated a hydrostatic head of about 120 cm. A whole surface bonded plexifilamentary sheet disclosed in International Patent Publication No. WO 98/07905 had a Gurley Hill Porosity of about 3.6 seconds and a hydrostatic head of about 55 cm. However, a greater degree of breathability is desirable for apparel fabrics and a far greater degree of air permeability is required of sheets to be used as air filter media.
Gurley Hill Porosity is a measure of the number of seconds that it takes to pass a fixed quantity, of air maintained at a certain pressure, through a sheet. The lower the Gurley Hill Porosity (measured in seconds), the greater the air permeability of the material. The Gurley Hill Porosity scale is generally used to quantify the porosity of materials with relatively low air permeabilities. The air permeability of more porous materials is generally measured in terms of Frazier permeability, which measures the volume of air at a given pressure that will pass through a given area sheet material. For plexifilamentary sheet materials of less than 3 oz/yd2, a Frazier Permeability of 2 ft3/min/ft2 corresponds to a Gurley Hill Porosity of about 3.1 seconds.
In order to provide more comfortable apparel fabrics and more breathable filters made from plexifilamentary sheet materials, there is a need for a plexifilamentary sheet material that demonstrates a Gurley Hill Porosity of less than 2 seconds while maintaining good liquid barrier properties. There is a need for a sheet material suitable for use in protective apparel that, at a given basis weight, has strength and barrier properties at least equivalent to that of the Tyvek® spunbonded olefin nonwoven fabric currently used for protective garments, but that also has significantly improved breathability to enhance the thermal comfort of protective apparel made of the material.